Ferrites which have the structure of the mineral spinel and the nominal atom composition MFe.sub.2 O.sub.4, where M is at least one divalent metal ion, are of considerable commercial interest. Of special interest for many uses, for example, filters and transformer cores, are manganese zinc (MnZn) ferrites because they can be manufactured with small low frequency losses, high permeability and low temperature and time dependence of desired properties, such as permeability. For MnZn ferrites, M is often Fe.sub.y Mn.sub.x Zn.sub.1-x-y, x and y both greater than 0 and less than 1. The presence of the divalent iron ions gives the MnZn ferrite the desired magnetic properties such as permeability and temperature coefficient of permeability. This formula represents the composition of the basic manganese zinc ferrite but because of a desire to obtain additional specific desired properties, other cations, such as titanium, calcium or silicon, are frequently added and the nominal composition of M will vary accordingly from the values cited.
The production of ferrites with specified properties and compositions has proven to be a difficult task and much effort has been directed toward developing simpler methods of ferrite fabrication. A commonly used method for ferrite fabrication involves high temperature solid-state reactions between the oxides or carbonates of the ferrite cations. The process begins with thorough mixing of very fine particles of, for example, oxides or carbonates of the cations. The remainder of the process involves calcining, milling, granulating, pressing and sintering the mixture. The last step proceeds at a relatively high temperature, typically between 1100 degrees C. and 1400 degrees C., after which the ferrite is cooled to the ambient temperature.
Those people concerned with MnZn ferrites have long realized that fabrication of MnZn ferrites with the desired magnetic properties, such as low temperature dependence of permeability, has required careful regulation of the oxygen partial pressure in the atmosphere during both the sintering and cooling steps. Control of the oxygen partial pressure during the latter step has been found to be especially crucial if the desired magnetic properties are to be obtained. Control has generally required not only monitoring and regulating the oxygen partial pressure of the atmosphere, but also varying the oxygen partial pressure as the temperature changes.
Regulation of the oxygen partial pressure in the atmosphere during cooling has been found necessary both to regulate the amount of Fe.sup.2+ in the ferrite and to prevent the precipation of hexagonal phases containing Fe.sup.3+ and Mn.sup.3+. The effects on the ferrite of not regulating the oxygen partial pressure during sintering and cooling include the undesirable properties of a greatly reduced permeability and a greatly increased negative temperature coefficient of permeability. The latter property is especially deleterious as it makes the design of devices that must operate over an extended temperature range difficult.